The invention relates to electrical contacts for use in low-voltage switchgear, the contacts being of the type which is based on silver, tin oxide and other additives. The invention relates also to a method for making contacts from this material, as well as to the contacts so made.
For low-voltage power switchgear, e.g. in contactors or automatic switches, contact materials based on silver metal oxides (AgMeO), in particular AgCdO, have proved advantageous. Since cadmium is known to be toxic and since CdO is given off during burnoff of the contact studs, it has been sought to replace the CdO by other metal oxides. These alternate materials should have the same advantageous characteristics as known AgCdO material, i.e. should have equally little arc burnoff, low welding load, and, in particular only slight heating under continuous current conduction.
Cadmium has been replaced by tin or zinc. However, AgSnO.sub.2 and AgZnO contact materials do not, on the whole, match the high-grade properties of AgCdO contact studs. In particular, when AgSnO.sub.2 is used as an alternative material for AgCdO it is found that, because of the higher thermal stability of SnO.sub.2, AgSnO.sub.2 has a higher transition resistance after switching load than does AgCdO, due to formation of oxide cover layers. This then causes contact studs to overheat switchgear, which may cause damage. On the other hand, AgSnO.sub.2 contacts have less burnoff than do AgCdO contacts, resulting in longer contact life. For this reason the size of the needed contact studs can advantageously be reduced by comparison with AgCdO, whereby a considerable saving of silver is achieved.
U.S. Pat. No. 4,330,330 describes a AgSnO.sub.2 -based material in which by addition of tungsten trioxide (WO.sub.3) the overtemperature is lowered as compared with pure AgSnO.sub.2 ; in addition, U.S. Pat. No. 4,341,556 proposes as a further additive bismuth oxide (Bi.sub.2 O.sub.3), which is said to improve the welding load without increasing the contact resistance.
U.S. Pat. No. 4,410,491 states that these overtemperatures can also be improved by adding molybdenum oxide (MoO.sub.3) and/or germanium oxide (GeO.sub.2). However the addition of MoO.sub.3 impairs the burnoff properties of AgSnO.sub.2 to such an extent that the life of the contacts is reduced far below that of AgCdO. While these disadvantages do not occur with the use of GeO.sub.2, the contact stud becomes more expensive because the price of germanium oxide is a multiple of the price of silver. This largely eliminates the economic advantage of the use of AgSnO.sub.2, namely a saving of silver because of the favorable burnoff properties as compared with AgCdO.
One object of the invention is therefore to develop an electrical contact material based on silver and tin oxide in which, by admixture of further additives, overtemperature is lowered as compared with the known AgSnO.sub.2 material without reducing the life of the contact or excessively increasing the cost of the material.